The number and quality of gases and particles being released from combustion processes are analyzed in different ways, both because of environmental decrees and for the purposes of process regulation. From an analysis standpoint, some challenging characteristics are associated with combustion processes, one of the most significant ones being the temperature. For example, the internal temperature of a chimney of a power plant near the reactor can be even much more than a thousand degrees Celsius. Moreover, even rather strongly corrosive agents and a plurality of various particles can flow in the chimney. Introducing a complicated, susceptible electronic measuring device into conditions such as these is impossible in practice due to durability of the materials alone, not to speak of the particle and other materials being accumulated on all surfaces and clogging the device very fast. So usually, the analysis of particles and gases is based on sampling and transporting the sample typically in a cooled and diluted form to be analyzed outside the process being measured.
In sampling it is very important that the sample is cooled and “extinguished” as fast as possible after taking from the process. As a result of various chemical and physical phenomena, the molecules and particles of a sample to be led freely out of the process e.g. along a simple steel pipe tend to change as the temperature drops and the gas is allowed to come into free contact with the gradually cooling walls of the pipe. Herein, the term “extinguishing” generally refers to stopping the chemical and physical processes such as these. The extinguishing is achieved by mixing the sample e.g. with cooled nitrogen or with some other inert gas. At the same time, the sample is diluted to suit the analysis.
For sampling, in which the sample is introduced from a sample space into a sampler and diluted as near as possible to the sample space prior to introducing it further into an analysis device, several solutions are known. As the outermost part, a typical sampler is provided with a tip part projecting into the sample space, through which the flow of sample is introduced into the sampler. The tip part may be a simple one, possibly a metal tube bent to be curved. After the tip part, a sampler is usually provided with a dilution part having a sample channel formed by a porous wall, the sample channel being enclosed by a dilution gas space formed by an impermeable wall. Dilution gas is introduced into the dilution gas space while maintaining in the dilution gas space a pressure higher than that in the sample channel. The pressure difference makes the dilution gas flow through the porous wall into the sample space, thereby diluting and cooling the sample. The circumference of the dilution gas space may also be provided with a cooling agent space for adjusting the temperature of the dilution gas as desired.
These structures have one very significant drawback. A dilution gas which is cooler than the sample and, especially, the walls of the sample channel and dilution space, attached by their ends to the tip part, cool the metal tube of the tip part in the vicinity of the orifice situated on the side of the dilution part thereof. An inner surface of a metal tube that is cooler than the sample acts as an efficient substrate for condensation of gases and accumulation of particles. A material remaining on such a cooler surface means losses of the constituents to be examined in the sample, which results in an erroneous analysis result. On the other hand, a material that has been accumulated on a cool surface can, at some later point, at a higher temperature, come again unstuck, thereby causing distorted results when analyzing the sample. The phenomenon described is the stronger the higher is the temperature difference between the sample space and the dilution gas. The liquid filled cooling agent space possibly included in the sampler and enclosing the dilution gas space enhances the cooling described even more. In typical solutions, the situation is partly also impaired by the dilution gas that hits the flow of sample perpendicularly from the side, the swirling of the sample flow caused by which dilution gas adds to the possibilities of the hot sample gas of getting into contact, at the beginning of the dilution part, with the wall of the sample channel that is cooler than the sample. In that case, there is a risk of material condensing from the sample, besides the tip part, also in the wall of the sample channel.
One prior-art solution of the kind described above is presented in patent publication U.S. Pat. No. 6,021,678 A. The basic problems are the same as in other known solutions. In this solution, too, the dilution gas is e.g. introduced, perpendicularly from the side, into a sample channel serving as an extension to the tip part and having an inner diameter of the same size with it. Thus, there is inevitably swirling at the beginning of the sample channel. As it is impossible in that case to efficiently prevent the sample from getting into contact with the wall of the cool sample channel, the publication describes how in the arrangement, preferably, the dilution gas to be introduced into the dilution gas space is kept at a higher temperature than the condensation temperature. Contrary to what one normally aims to do, one does not try to cool the sample quickly after taking from the sample space, but it is kept at a high temperature. This leads easily to the problems with the changes in the sample as described above. However, in several solutions, cooling is necessary prior to the analysis, so the solution just postpones the problems associated with the cooling without solving them. The heating of the dilution gas also makes the equipment more complicated.
Known from patent publication US 2005/0236040 is a solution in which a sample channel extending into a diluter is enclosed by a concentric jacket-like protection gas channel, into which hot inert gas is introduced. Thus, the sample is introduced into the diluter so that is enclosed by hot protection gas. In order that the sample flow and the wall of the sample channel would be maintained at the temperature of the sample space all way to the diluter, the protection gas channel must begin already on the side of the sample space. Providing a device with this kind of protection gas channel with all the necessary valves and heaters is a very complicated and expensive solution. In addition, the higher is the temperature of the sample space, the more difficult it is in practice to keep the high temperature of the sample and the protection gas all the way from the sample space to the diluter.